INTRODUCTION: Estradiol induces an early (one hour) increase in the translational efficiency of uterine ribosomes in ovariectomized mature rats which results in a specific activation of the peptide elongation rate (PER). The effect is related inversely to the amount of estradiol receptor which is bound to the ribosome and is caused by reversal of the activity of a ribosome associated factor which inhibits the attachment of selected amino acids to tRNA. SPECIFIC AIMS: 1) To characterize the ribosome-associated estrogen receptor as it relates to the cytosolic receptor and the inhibitor of the ribosomal PER, 2) To purify and characterize the inhibitor of the PER, 3) To determine the specific action of this inhibitor on tRNA which renders the tRNA incapable of supporting protein synthesis and 4) To characterize a "seasonal" change which results in the loss of the uterine PER response to estrogen and to a reduction in the levels and properties of the estrogen receptor system during the winter months. METHODS: The peptide elongation rate (PER) is measured as the ratio of the protein synthesis rate (PSR) divided by the nascent peptide content (NPC) by isolated uterine ribosomes. PSR is determined as the rate of incorporation of 3H-leucine into protein and the NPC is determined by the amount of 3H-puromycin which can react with nascent peptides on the ribosomes. Ribosomal and cytosol estrogen binding proteins are measured using 3H-estradiol under exchange (30 degrees, 1 hr) and non-exchange (2 degrees, 18 hr) conditions. SIGNIFICANCE: Activation of the translational capacity of pre-existing uterine ribosomes by estradiol will decrease the time required to translate "early mRNAs" which may code for proteins which have unique regulatory functions, and this may modify or attenuate the overall uterine responsiveness to the steroid.